pckmn pckmn - 1 month ago 35
Scala Question

How can I calculate exact median with Apache Spark?

https://spark.apache.org/docs/0.7.0/api/core/spark/api/java/JavaDoubleRDD.html

This page contains some statistics functions(mean, stdev, variance etc.) but it does not contains median. How can I calculate exact median ?

Thanks

Answer

You need to sort RDD and take element in the middle or average of two elements. Here is example with RDD[Int]:

  import org.apache.spark.SparkContext._

  val rdd: RDD[Int] = ???

  val sorted = rdd.sortBy(identity).zipWithIndex().map {
    case (v, idx) => (idx, v)
  }

  val count = sorted.count()

  val median: Double = if (count % 2 == 0) {
    val l = count / 2 - 1
    val r = l + 1
    (sorted.lookup(l).head + sorted.lookup(r).head).toDouble / 2
  } else sorted.lookup(count / 2).head.toDouble